It has been conventionally known that the oscillation wavelength of a semiconductor laser (hereinafter referred to merely as an LD) greatly varies due to a slight temperature variation and injection current variation.
It has been proposed that, by making injection current control and temperature control while regulating a wavelength reference with the use of the Fabry-Perot resonator having a periodic transmission spectral characteristic, the oscillation wavelength of LD is locked to a predetermined mode of the Fabry-Perot resonator so that the oscillation wavelength is stabilized.
At that time, in view of the necessity to oscillate LD near the predetermined mode of the Fabry-Perot resonator it is necessary to stabilize the temperature of LD as the pre-process of the wavelength stabilization. Further it is necessary to stabilize the temperature of the Fabry-Perot so as to obtain the characteristic of a stabilized transmission spectral characteristic.
In order to stabilize the temperature of an LD 4 provided in an LD resonator 2, an LD temperature sensor 6 is provided near LD 4, as shown in FIG. 15, so as to detect the temperature of LD 4. Based on a temperature detection signal output from the LD temperature sensor 6, a temperature controller 8 controls a drive current in a Peltier element 10 so that the temperature of LD 4 is maintained at a predetermined temperature. It is to be noted that a fin member 12 for heat dissipation is provided at a Peltier element 10.
As shown in FIG. 16 in the neighborhood of a solid type etalon 16 provided in an etalon holder 14, an etalon temperature sensor 18 for detecting the temperature of the etalon 16 is provided so as to stabilize the temperature of the Fabry-Perot resonator. A temperature detection signal is output from the etalon temperature sensor 18 and, based on the temperature detection signal, a temperature controller 22 controls a drive current in a Peltier element 24 so that the temperature of the etalon 16 is maintained at a predetermined level. It is also to be noted that the Peltier element 24 is equipped with a fin element 26 for heat dissipation.
The following two conditions are required to achieve the wavelength stabilization of LD.
(1) LD is oscillated in the neighborhood of a predetermined mode of the Fabry-Perot resonator.
(2) A Fabry-Perot resonator is achieved which has a stabilized reference without being susceptible to an influence from an ambient temperature variation.
The conventional method poses the problem that, if the ambient temperature varies even in the case where the temperature of LD is stabilized, it is difficult to oscillate LD near the Fabry-Perot resonator (the problem encountered due to the ambient temperature variation). Further, there is also the problem that the LD's wavelength, though slightly with the passage of time, drifts even if the injection current and temperature are constant (the problem encountered with the passage of time).
Explanation will be given below about the following two problems.
&lt;The Problem Encountered From the Ambient Temperature Variation&gt;
As shown in FIGS. 15 and 16, the LD temperature sensor 6 and etalon temperature sensor 18 do not have their measuring points aligned with the mounting positions of LD 4 and etalon 16, problems as set out below arise in the case where ambient temperature in LD 4 and etalon 16 varies in the LD holder 2 and etalon holder 14, respectively.
The problem under consideration will be explained below with reference to FIG. 17. In FIG. 17, the abscissa denotes a thermal resistive value with the position of the LD temperature sensor 6 (etalon temperature sensor 18) set as a reference and the ordinate, the temperature involved.
As shown in FIG. 17, the temperature (T.sub.A) of the LD temperature sensor 6 (etalon temperature sensor 18) is stabilized to the set reference level (T.sub.A0) and, at that time, the temperature (T.sub.L) of LD 4 (etalon 16) and ambient temperature (T.sub.B) are assumed to be on the solid line in FIG. 17.
Now let it be assumed that the ambient temperature (T.sub.b) is lowered. Since normal operation is involved under temperature control, the LD temperature 6 (etalon temperature sensor 18) is held at the set reference level (T.sub.A0).
In actual practice, however, LD 4 (etalon 16) will be stabilized to a temperature lower by .DELTA.T.sub.L than the temperature (T.sub.L).
FIG. 18 shows a corresponding temperature control flow. The temperature (T.sub.A) is detected with an LD temperature sensor 6 (etalon temperature sensor 18)-S1. Based on that temperature detection signal, the temperature controllers 8, 22 compute a deviation (.DELTA.) from the set reference temperature (T.sub.A0)-S2. A Peltier current (I.sub.PL) is computed from that calculated value-S3 and a drive current in the Peltier elements 10, 24 is controlled-S4. In FIG. 18, P denotes a proportional coefficient; I, an integral coefficient; and D, a derivative coefficient.
From this, the following defects arise.
First, LD 4 oscillates with a wavelength greatly deviated from a predetermined mode of the etalon 16, that is, a wavelength reference, due to the variation of the ambient temperature. As a result, it is not possible to achieve the locking of the wavelength involved or the wavelength will fail to be locked to other modes.
Second, the spectral characteristic of the etalon 16, that is, the wavelength reference, will be varied due to the variation of the ambient temperature.
&lt;The Problem Encountered With the Passage of Time&gt;
The oscillation wavelength of LD varies with the passage of time as shown in FIG. 19 (OPTRONICS (1992) No. 11 [Feature of LD Heterodyne Light Source and How to Use It]; YAMAURA, ISHIBAI, HIRANO--HOYA Co. Ltd. That is, a straight line as shown in FIG. 12 varies with the passage of time and the oscillation wavelength of LD is deviated from .lambda..sub.0 even if the temperature of LD is T.sub.1 (.degree. C.) and injection current of LD is il. As a result, even if the temperatures of LD and injection current of LD are set to a predetermined value, a desired oscillation wavelength fails to be obtained and the wavelength involved can be locked to a different wavelength.
The present invention is achieved so as to solve these problems. It is accordingly the object of the present invention to provide a wavelength stabilizing apparatus for stabilizing an oscillation wavelength without being affected due to a variation of ambient temperature and a variation of an LD with the passage of time.